Ultrahigh surface area materials and methods of making same

ABSTRACT

In one embodiment, a surface has a laser-beam machined area including an array of micro-sized conical pillars that are arranged in orthogonal rows and columns across the surface and that extend upward, the conical pillars defining deep troughs between them that are configured to absorb electrons, electromagnetic radiation, or both, the conical pillars tapering from relatively wide bases to pointed tips, the conical pillars comprising outer surfaces that are covered with a plurality of nanoparticles.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application Ser.No. 62/578,881, filed Oct. 30, 2017, which is hereby incorporated byreference herein in its entirety.

BACKGROUND

Electron spectroscopy is an analytical technique that is used to studyelectronic structure and its dynamics in atoms and molecules. Generallyspeaking, an excitation source ejects electrons from an inner-shellorbital of atoms of a material. Detecting photoelectrons that areejected by x-rays is called x-ray photoelectron spectroscopy (XPS) orelectron spectroscopy for chemical analysis (ESCA). Detecting electronsthat are ejected from higher orbitals to conserve energy during electrontransitions is called Auger electron spectroscopy (AES).

In any type of spectroscopy, noise in the spectrum diminishes thesensitivity of the spectrometer. This becomes a problem when highsensitivity is needed, such as when trying to identify elements at atrace level in the sample. One significant source of noise in electronspectroscopy is scattering of secondary electrons on the inside surfacesof the spectrometer. While various attempts have been made to createsurfaces that suppress secondary electron scattering, such as theformation of saw-tooth grooves and deposition of silicon thin films,none have met with great success. It therefore can be appreciated thatit would be desirable to have materials that incorporate surfaces thateffectively suppress electron scattering.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure may be better understood with reference to thefollowing figures. Matching reference numerals designate correspondingparts throughout the figures, which are not necessarily drawn to scale.

FIG. 1 is an image of a copper gasket that was laser-beam machined inseveral discrete areas. These areas are relatively dark in the image.

FIGS. 2A-2D are scanning electron microscope (SEM) images of alaser-beam machined area of the copper gasket of FIG. 1 shown at 50×,76×, 400×, and 800× magnification, respectively. A secondary electronemission waveform is overlaid in FIG. 2B to illustrate secondaryelectron absorption of the laser-beam machined area.

FIG. 3 is an image of a further copper gasket that was laser-beammachined in two discrete areas. These areas are relatively dark in theimage.

FIGS. 4A-4D are SEM images of a laser-beam machined area of the coppergasket of FIG. 3 (at the 12 o'clock position) shown at 50×, 200×, 250×,and 10,000× magnification, respectively.

FIG. 5A is the image of the copper gasket of FIG. 3 labeled to identifya location at which energy dispersive spectroscopy (EDS) was performed.

FIG. 5B is a graph that shows the spectrum that was obtained during theEDS identified in FIG. 5A.

FIGS. 6A and 6B are schematic views of components of a cylindricalmirror analyzer (CMA) of an electron spectrometer, including an outercylinder whose inner surface can be provided with a laser-beam machinedarray of micro-sized conical pillars.

FIGS. 7A-7D are SEM images of an interface between a non-machined areaand a laser-beam machined area of the copper gasket of FIG. 3 shown at50×, 50×, 50×, and 70× magnification, respectively. A secondary electronemission waveform is overlaid in FIGS. 7B and 7D to illustrate secondaryelectron absorption of the laser-beam machined area.

FIGS. 8A-8D are SEM images of a further interface between a non-machinedarea and a laser-beam machined area of the copper gasket of FIG. 3 shownat 100×, 90×, 92×, and 94× magnification, respectively. A secondaryelectron emission waveform is overlaid in FIGS. 8B-8D to illustratesecondary electron absorption of the laser-beam machined area. Thesefigures were defocused in order to obtain a better average along theline scan of the secondary electron emission.

FIG. 9 is a schematic side view of a portion of a laser-beam machinedarea that comprises a high-density array of micro-sized conical pillars.

DETAILED DESCRIPTION

As described above, it would be desirable to have materials thatcomprise surfaces that effectively suppress electron scattering.Disclosed herein are examples of such materials and surfaces. In someembodiments, a surface of a material is laser-beam machined so as tocomprise an array of micro-sized conical pillars that extend upward fromthe surface of the material. In some embodiments, the conical pillarsare covered with nanoparticles. Together, the size and shape of theconical pillars and the presence of the nanoparticles greatly increasethe surface area of the material's surface, which enables the surface toabsorb electrons and thereby suppress electron scattering. Notably, thelaser-beam machined surfaces can be used in applications beyond electronabsorption. For example, the laser-beam machined surfaces can, in someembodiments, be used to absorb electromagnetic radiation. In such cases,the surfaces can be used in a variety of other applications, such aslight-absorption and stealth applications.

In the following disclosure, various specific embodiments are described.It is to be understood that those embodiments are exampleimplementations of the disclosed inventions and that alternativeembodiments are possible. All such embodiments are intended to fallwithin the scope of this disclosure.

Disclosed herein are ultrahigh surface area materials that are formedusing laser-beam machining. Such materials were first developed by theinventor while seeking a means to absorb electrons to minimize secondaryelectron scattering in electron spectroscopy applications. Duringexperimentation, the surface of oxygen-free high-temperature copper(OFHC) was machined using a 30 megawatt peak power pulsed neodymiumvanadate laser (Nd-YVO₄). The laser had a 5× lens with a spot size of 16μm. The laser had an infrared wavelength of 1064 nm, a pulse width of 15picoseconds, and a pulse repetition rate of up to 100 kHz. The averagepower of the laser can be adjusted to be as high as 10 W. The laser wasset to 3 W with a linear scan speed of 50 mm/s, which was consideredoptimal. In the experiment, a 2¾ in. diameter, 1 mm thick OFHC gasketshown in FIG. 1 was used as a substrate to simulate the inner surface ofan outer cylinder used in a cylindrical mirror analyzer (CMA).

The laser system was used to machine an orthogonal array of micro-sizedconical pillars (micropillars) having deep troughs formed between themthat act as Faraday cups configured to absorb secondary electrons. Toproduce the array, the scanning stage of an nScript 3D laser printer wasprogrammed to scan the surface of the OFHC gasket along orthogonallyarranged rows and columns to create an orthogonal grid pattern. Thelines in both the rows and the columns were spaced from each other by 50μm, forming a grid with 50×50 μm spacing. Experiments were performed todetermine the laser power needed to cut deep grooves in the gasket.Three different power settings were used: 0.5, 1.5, and 3 W. It wasdetermined that 3 W was needed to form grooves in the OFHC material,which caused dark patches to appear in the copper, as shown in FIG. 1.In that figure, three laser-beam machined areas are labeled, 3w1p, 3w3p,and 3w6p. Each of these areas was formed using 3 W of power, and with 1,3, and 6 repeated passes of the 50×50 μm crosshatch pattern. Accidentaloverlapping of two adjacent laser-beam machined areas formed a darkline, which is identified in FIG. 1. These results indicate that greaterlaser power was needed to form deep grooves in the OFHC material andproduce secondary electron-absorbing structures. As shown in FIG. 1, the3w6p area appeared darker, as expected, because deeper grooves wereformed in that area.

FIGS. 2A-2D are scanning electron microscopy (SEM) images of the darkpatch shown in FIG. 1. FIG. 2A shows the dark overlapping area at 50×magnification. FIG. 2B shows the overlapping area at 76× magnificationand includes an overlying secondary electron emission waveform obtainedusing a beam energy of 3 keV. As can be appreciated from FIG. 2B, therewas a large decrease in secondary electron emission in the darkoverlapping area. FIGS. 2C and 2D are 400× and 800× magnificationimages, respectively, of the overlapping area and reveal micro-sized,flat-topped pyramids that resulted from the laser-beam machining.

In a later experiment, a further OFHC gasket, shown in FIG. 3, waslaser-beam machined using a line spacing of 25×25 μm. The laser powerwas set to 3 W and 6 repeated passes were performed to form a 10×10 mmlaser-beam machined area. This area is visible at the 12 o'clockposition on the gasket in FIG. 3. The 25×25 μm laser scan spacingproduced a high-density array of micro-sized conical pillars(micropillars). In this example, the conical pillars had an area densityof 1.6×10⁵ per square centimeter. Because of the troughs formed betweenthe conical pillars, electrons that strike the array have a lowprobability of escape. Accordingly, the array functions as an electronsuppressor. In addition, given that the array is dark in color, itfurther absorbs light, as well as other forms of electromagneticradiation. Specifically, light that enters the troughs between theconical pillars undergo multiple scattering that limits the amount oflight that can escape the array.

FIG. 4A is a SEM image at 50× magnification of an inside edge of thelaser-beam machined area formed on the OFHC gasket. This edge is visibleas the bottom corner of the black rectangle located at the 12 o'clockposition on the gasket in FIG. 3. At this location, the corner of the10×10 mm scan area extended beyond the inside edge of the gasket. As isapparent in FIG. 4A, the laser-beam machined area is comprised of ahigh-density array of micro-sized conical pillars. The SEM image in FIG.4B shows a portion of this area at 200× magnification. In this example,the majority of the conical pillars of the laser-beam machined area wereapproximately 100 μm tall. As a consequence, the troughs between theconical pillars were approximately 100 μm deep. As is visible in FIG.4B, the conical pillars along the left edge of the area, however, wereapproximately 200 μm tall. This height difference was due to theincreased laser dose the gasket received in that location from the laserstopping at the edge of the area and changing scan direction for eachline that was scanned. FIG. 4C shows the conical pillars at 250×magnification and FIG. 4D shows the tip of one of the conical pillars at10,000× magnification. As can be appreciated from FIG. 4D, each conicalpillar has a pointed tip and the outer surface of the pillars arecovered with a plurality nanoparticles, such as nanospheres, that rangein size (e.g., diameter) from approximately 1 nm to 1 μm. Significantly,these nanoparticles are not deposited or otherwise added to the conicalpillars. Instead, the nanoparticles form as a natural consequence of thelaser-beam machining. Due to the 25 μm line spacing, the bases of thepillars had width dimensions (e.g., diameters) of approximately 25 μm.The pointed tips of the conical pillars were approximately 1 to 2microns in radius.

Energy dispersive spectroscopy (EDS) was performed in the center of the10×10 mm laser-beam machined area, as identified in FIG. 5A. Table 1comprises the results of the quantitative elemental EDS analysis andshows the laser-beam machined area to be 98% copper with 2% oxygen.

TABLE 1 SEM EDS Quantitative Elemental Analysis of Laser-Beam MachinedArray Intensity Error Low High Atomic Element Line (c/s) 2-sig keV keV %Conc. Units Oxygen Kα 82.02 2.574 0.471 0.579 7.545 2.013 wt % Copper Kα1306.40 9613 7.949 8.147 92.455 97.987 wt % 100.00 100.00 wt % Total kV16 Tilt TOA LT 30° 60° 60s 98% Copper Total

Experiments were also performed to test the ability of an outer cylinderof a CMA of an electron spectrometer in suppressing secondary electronscattering. FIGS. 6A and 6B illustrate an example of such an outercylinder as used in an electron spectrometer. As shown in these figures,the outer cylinder comprises an inner surface that electrons from asample strike during operation of the equipment. It is this surface thatcan be laser-beam machined to form an array of micro-sized conicalpillars that suppress secondary electron scattering. In order tosimulate electrons striking the inside surface of an outer cylinder of aCMA, an OFHC substrate was mounted in a scanning electron microscopetilted at 42.3 degrees to approximate the angle of attack of theelectrons striking the inside of the outer cylinder.

A focused electron beam was first used to find the area of interest andcollect a SEM image. Next, the beam was purposely defocused to simulatediffuse low-current electrons as would be the case for electronsstriking the outer cylinder. FIG. 7A shows a focused SEM image of acorner of the dark rectangle shown in FIG. 3. As can be observed in FIG.7A, the laser-beam machining produced a large number of flat-toppedconical pillars in the OFHC with deep grooves or troughs between them.The deep grooves/troughs act to absorb electrons while the flat topsenhance electron emission.

In FIGS. 7B-7D, SEM images were captured after the laser beam wasdefocused to the point where the conical pillars could no longer beresolved by the electron beam. This indicates that the beam diameter waslarger than the conical pillars and thus the effect is averaged overseveral pillars. The laser dose in FIGS. 7A and 7B was 3 W with one passof the laser. The electron line scan waveform starts on the left showinga large increase in baseline electron emission as the electron beammoves onto the laser-beam machined area. The laser dose in FIGS. 7C and7D was 3 W with three repeated passes of the 50×50 μm crosshatch. Thisproduced deeper grooves and increased electron absorption, as shown inthe waveform in FIG. 7D. It is important to note the brightness andcontrast gain on the secondary electron defector used to collect thesewaveform measurements must remain unchanged

As seen in FIGS. 7A-7D, increasing the number of passes increased thedepth of machined trenches and reshaped the tops of the pillars fromflat, which increases electron emission, to a cone-shaped, which absorbselectrons. In FIGS. 8A-8D, the laser beam was set to a power of 3 W andwas scanned with six repeated laser passes and a 25×25 μm crosshatchpattern over a 10×10 mm area. A line scan across the OFHC and theconical pillar array interface was performed and the secondary electronoutput is displayed in FIG. 8 as a waveform superimposed on top of theimage. The beam was defocused in order to get a better average along theline scan of the secondary electron emission. This was repeated at threecommonly used beam energies used for Auger spectroscopy analysis in theCMA. FIGS. 8B, 8C, and 8D show the secondary electron emission waveformsfor 1, 2, and 3 keV electrons, respectively. As can be appreciated fromthese waveforms, the secondary electron emission was significantlyreduced when the electron beam entered the laser-beam machined array.Notably, the secondary electron suppression does not appear to depend onbeam energy. For all three energies, the relative secondary electrondrop was approximately the same. The lowest secondary electron emission,which produced brightness in these waveforms, occurred at the edge ofthe conical pillar array. It is at that edge where the laser beam hadthe longest residence time as it performed the scanning. As noted above,this occurred because the laser scans a short distance to move to thenext scan line and this results in the edges receiving larger laserresidence times. This creates deeper grooves and troughs, which wereapproximately 200 μm deep (see FIG. 4). As can be appreciated from theresults shown in FIG. 8, at beam energies between 1 and 3 keV (i.e., therange typically used in the CMA), there is little change in the abilityof the conical pillar array to suppress secondary electron emission.

In the foregoing discussion, specific embodiments of ultrahigh surfacearea surface materials have been described that are specificallyconfigured to absorb electrons within electron spectrometers. While thesurfaces are well suited for such an application, other applications arepossible. For example, laser-beam machined surfaces of the typedescribed herein can be used to absorb electromagnetic radiation,including light waves, microwaves, and radio waves, and one or moreparameters of the laser-beam machined array can be tuned for particularapplications. As examples, the materials used to form the array can bealtered and the physical parameters of the conical pillars of the arraycan be tuned for particular applications. Through the experiments, itwas determined that a high aspect conical shape performed well as anelectron absorber. In some applications, such as in an electrondetector, it is an advantage to have increased electron emission versusan unmodified surface. FIG. 7D shows a rectangular array oflaser-ablated OFHC squares at 50× magnification. The laser beam was thendefocused and a line scan was taken to plot the electron emissioncharacteristics. The electron emission waveform overlaid on thesecondary electron image in FIG. 7D shows increased emission as the beammoves on to the laser-beam machined area.

The above-described laser-beam machined arrays have many otherapplications beyond electron absorption. The arrays may be used insubstantially any application in which the surface area is to beincreased to have an effect on absorption and emission of other types ofradiation, for example, as a heat sink, an electromagnetic (EM)radiation sink, or a Faraday cage. The periodicity of themicrostructures may also be tuned to the desired wavelength to act as aresonant EM absorber. There will likely also be applications of thesemicrostructures chemical catalytic reactors, and for use as chemicalsensors. Coatings applied over these surfaces can have the surface areaaccurately controlled to tune chemical reaction rates within the rangeof a process. In some embodiments, the number of pillars in the array islimited by the laser beam diameter

FIG. 9 schematically illustrates a portion of a laser-beam machinedarray 10 of micro-sized conical pillars (micropillars) 12 that form anultrahigh surface area surface 14 on a substrate 16 that can comprisepart of substantially any component or device that is to incorporate anultrahigh surface area surface. As the array 10 is formed by laser-beammachining the surface 14 of the substrate 16, the substrate and thearray of micro-sized conical pillars 12 are made from a single unitarypiece of material. In some embodiments, the substrate 16 and the array10 (including the conical pillars 12) are made of a metal material,whether it be a pure or alloyed metal. As noted above, the substrate 14can be made of copper, such as OFHC. Other example electricallyconductive metals used in electron vacuum tubes and particleaccelerators include magnesium, stainless steels, beryllium copper,aluminum, brass, gold, platinum, palladium, iridium, and tungsten. Inother embodiments, the substrate 14 can be made of non-metal material,such as a polymeric or ceramic material. In some embodiments, thesurface area of a polymer paint or coating may be tuned by laserprinting a pattern such as that described above to enhance heat exchangeor for purposes of chemical activity, for example in the case of acatalyst or chemical sensor. Other example substrate materials and orcoatings may include refractory ceramics and plasma coatings such asaluminum oxide, magnesium oxide, and zirconia.

As depicted in FIG. 9, each conical pillar 12 is generally conical inshape and, therefore, each has a relatively wide base 18 and graduallytapers as the pillar is traversed upward from the base to a pointed tip20. While the tips 20 are “pointed,” as can be appreciated from imagessuch as that of FIG. 4C, it is noted that the tips can further beslightly rounded, at least when viewed at a highly magnified scale, asdepicted in FIG. 9. In some embodiments, each tip 18 can have a radiusof curvature of approximately 0.5 to 50 μm. As further shown in FIG. 9,the conical pillars 12 can taper from base 18 to tip 20 at an angle δ ofapproximately 1 to 20°. The bases 18 can have a width dimension W_(B)(e.g., diameter) that is approximately 1 to 50 μm and the tips 20 ofadjacent pillars 12 (along any given row or column in the array 10) canbe spaced a distance ST that is approximately 5 to 50 μm. Notably, asthe conical pillars 12 are each typically vertically aligned, thedistance ST typically corresponds to the spacing of the scan lines usedduring the laser-beam machining.

The height dimension of the conical pillars 12, P_(H), can range fromapproximately 0.1 to 200 μm, such as 50 to 150 μm, and the troughs 20formed between adjacent pillars can have similar depth dimensions T_(D).As noted above, the conical pillars 12 can include a plurality ofnanoparticles 22, which greatly increases the surface area of thepillars 12 and the array 10. As was also noted above, thesenanoparticles 22 can range in size (e.g., diameter) from approximately 1nm to 1 μm. The array 10 can have a high pillar density such that thereare numerous conical pillars 12 per unit area. In some embodiments, thearray has a pillar density of approximately 1.0×10⁴ to 1.0×10⁸ pillarsper square centimeter.

In addition to the parameters of the laser-beam machined array and itsconical pillars, the parameters of the laser-beam machining process canbe altered depending upon the application. In some embodiments, an arraycan be formed by scanning a laser beam having a spot size ofapproximately 1 to 20 μm and a wavelength of approximately 0.25 to 10 μmacross the surface of material that is to be provided with an ultrahighsurface area. The laser can be operated at an average power of 1 to 10 Wwith a pulse width of 0.01 to 100 picoseconds and a repetition rate ofapproximately 0.1 to 500 MHz. The laser beam can be scanned across thesurface at scan speed of approximately 1 to 100 mm/s with a scan linespacing of approximately 5 to 50 μm. Each line can be scanned with 1 to10 passes to achieve the desired machining depth.

What is claimed is:
 1. A method comprising: producing an ultrahighsurface area substrate by: forming a grid pattern of orthogonal rows andcolumns on a surface of a chosen substrate by laser-beam machining saidsubstrate, wherein said grid pattern includes an array of micro-sizedconical pillars extending from a body of the chosen substrate andtroughs defined between said conical pillars, and simultaneously withsaid forming, generating pluralities of nanoparticles at outer surfacesof said conical pillars by said laser-beam machining.
 2. A methodaccording to claim 1, wherein said conical pillars are tapered fromcorresponding pillar bases at the chosen substrate towards pointed tipsthereof.
 3. A method according to claim 1, wherein said forming includesshaping tips of said conical pillars to be rounded.
 4. A methodaccording to claim 1, wherein said forming includes configuring saidgrid pattern to absorb electrons, electromagnetic radiation, or both. 5.A method according to claim 1, wherein said generating pluralities ofnanoparticles does not include adding nanoparticles to said conicalpillars.
 6. A method according to claim 1, wherein said forming includesconfiguring the grid pattern as a Faraday cage.
 7. A method according toclaim 1, wherein said forming includes configuring the troughs asFaraday cups.
 8. A method according to claim 1, wherein the laser-beammachining includes scanning a laser beam across the chosen substrate ata scan speed of approximately 1 millimeter per second to 100 millimetersper second.
 9. A method according to claim 1, wherein the laser-beammachining includes scanning a laser beam across the chosen substratewith 1 to 10 passes along each row and column.
 10. A method according toclaim 1, wherein the laser-beam machining includes scanning a laser beamhaving a spot size of approximately 1 micron to 20 microns across thechosen substrate.
 11. A method according to claim 1, wherein thelaser-beam machining includes scanning a beam of laser light at awavelength of approximately 0.25 micron to 10 microns across the chosensubstrate.
 12. A method according to claim 1, further comprisingdelivering laser light with a power of approximately 1 Watt to 10 Wattsto the chosen substrate.
 13. A method according to claim 1, furthercomprising irradiating the chosen substrate with laser light carrying apulse having duration of approximately 0.01 picosecond to 100picoseconds.
 14. A method according to claim 1, further comprisingirradiating the chosen substrate with laser light with pulses at arepetition rate of approximately 0.1 MHz to 500 MHz.
 15. A methodaccording to claim 1, wherein said forming a grid pattern on a surfaceof the chosen substrate includes forming the grid pattern on a singleunitary piece of material to have a spatial density of said conicalpillars within a range from 1×10⁴ per square centimeter to 1×10⁸ persquare centimeter to suppress scattering of electrons at said chosensurface.
 16. A method comprising performing X-ray photoelectronspectroscopy or Auger electron spectroscopy with the use of theultrahigh surface area substrate produced according to the method ofclaim 1.